Abstract

578-nm yellow light with an output power of more than 10 mW was obtained using a waveguide periodically-poled-lithium-niobate crystal as a nonlinear medium for second harmonic generation, which is the highest output power at this wavelength using second harmonic generation of a solid state laser source without an enhancement ring cavity, to our knowledge. To achieve this result we made a high power 1156-nm external-cavity diode laser with the maximum output power of more than 250 mW. This system is expected to be an excellent alternative to the system using the sum-frequency generation with the advantage of simplicity and cost-effectiveness, and will be used as a clock laser of the ytterbium optical lattice clock with robust and reliable operation.

1. Introduction

Although laser light sources were invented nearly five decades ago and are widely used in a variety of scientific and technical applications, some wavelength regions are still very hard to reach. Especially, it has been difficult to generate yellow light directly using solid-state laser sources. Thus, dye lasers have been used for this wavelength range [1

]. In the past two decades, the technical developments of diode-laser-pumped solid-state lasers and fiber lasers in the wavelength region from 1000 nm to 1500 nm have made it possible to generate yellow light using second harmonic generation (SHG), sum frequency generation (SFG), or optical parametric oscillators. Solid-state lasers have experimental advantages over dye lasers in realizing stable narrow linewidth yellow lasers more easily with stable operation.

Among these applications, a narrow linewidth 578-nm laser source is required to probe the clock transition of ytterbium atom confined in an optical lattice, which is a candidate for an optical clock with less than 10−16 frequency uncertainty [1

] has experimental merits in narrowing the laser linewidth as the initial linewidths of both infrared lasers are about 1 kHz, if monolithic-cavity solid-state lasers are used. Hong et al. obtained 22-mW 578-nm-radiation with the SFG scheme using a waveguide periodically-poled-lithium-niobate (WG-PPLN) device, when the input powers of the 1319-nm laser and the 1030-nm laser were 40 mW and 25 mW, respectively [8

], and its experimental scheme is much simpler than SFG system, because only one laser is used. Nevsky et al. obtained 3-mW 578-nm-radiation by the SHG of an external cavity InGaAs quantum dot laser at 1156 nm [10

]. They used an external ring cavity to enhance the SHG output power using a PPLN bulk crystal. Also, 2.4-mW yellow light at 578 nm was previously obtained in our group by the SHG using a WG-PPLN with the conversion efficiency of 6.5% [11

]. In that research, an optical injection-locking technique had been adopted to amplify the optical power of the 1156-nm master laser to 40 mW.

In this paper, we obtained the 578-nm yellow laser output power of more than 10 mW using a WG-PPLN as a SHG material, which is the highest output power at this wavelength using SHG of a solid state laser source without an enhancement ring cavity, to our knowledge. Although there are one commercial system (Toptica, DL SHG pro) based on an enhancement ring cavity, which guarantees the output power of 12 mW at 578 nm, our system has an advantage of simplicity, as the SHG is obtained by single pass of 1156-nm laser and the condition for the temperature stability of the frequency-doubling crystal is not so severe as in the in-cavity case. Also, considering that our system is planned to be used as a clock laser of the ytterbium optical lattice clock, this simple scheme is preferred, because a ring cavity for the frequency doubling can bring technical issues, such as possible amplitude modulations, in stabilizing the frequency to an ultra-stable super-cavity. Meanwhile, although not essential, the high output power capability at 578 nm is preferred in probing the ytterbium lattice clock transition, because initial observation of the clock transition becomes much easier due to the power broadening. Also, the clock laser intensity should be high enough to observe the red- and blue-sideband of the clock transition of ytterbium atom confined in an optical lattice, which is used in measuring the temperature of the lattice trap [12

]. In addition, as the 578-nm clock laser should be divided into three parts and delivered through single-mode optical fibers to the super-cavity for the linewidth reduction, to the atom chamber for the spectroscopy, and to the optical frequency comb for the absolute frequency measurement with fiber-noise-cancellation setups [13

] and power-stabilization setups in each part, the output power should be high enough for the robust and reliable operation of the ytterbium lattice clock. Furthermore, the more power is needed in the magnetically induced spectroscopy for the doubly forbidden clock transition of the bosonic ytterbium isotopes [2

]. The output power of 10 mW is expected to be sufficient for these purposes. To achieve this result, we made a high power 1156-nm external-cavity diode laser (ECDL) with the maximum output power of more than 250 mW. Compared with our previous system [11

], the output power at 1156 nm was improved by more than five times with much simpler experimental setup. The experimental detail of building the high power ECDL and the performance of SHG with this laser system will be described in the following sections.

2. High Power 1156-nm ECDL

The experimental setup for the yellow light generation is shown in Fig. 1

]. The back facet of the GC has highly reflective coating (reflectivity > 99%), and the front facet is anti-reflection (AR)-coated (reflectivity < 0.1%). The output power of the GC as a function of the operating current is shown in Fig. 2(a)

Fig. 2 (a) Output power of a gain chip without optical feedback by a diffraction grating (open black circles) and that of an ECDL with 5% feedback by a diffraction grating (filled red circles) as a function of the operating current, (b) Fiber-Mach-Zehnder interferometer signal when the laser frequency was scanned.

with open black circles. The output of the GC was collimated using an AR-coated aspheric lens (Thorlabs, C230TME-C). The focal length and the numerical aperture of this lens were 4.51 mm and 0.55, respectively. The collimated beam has an aspect ratio of about 6:1 with the length along the long axis of about 5 mm. The polarization direction was parallel to the short axis of the collimated beam. About 5% of the collimated beam was diffracted in the first order and fed back into the GC using a 12.7 × 12.7 × 6.0 mm3 diffraction grating (Edmund, W48-464) with the light polarization direction parallel to the grating rules. This is a 1200-groove ruled grating with a blaze wavelength of 400 nm, and the grating incidence angle was about 44 degrees at 578 nm for the Littrow configuration. A square mirror (12.7 × 12.7 × 3.0 mm3) was used as a compensation mirror to tune the optical frequency without changing the direction of the output beam [15

]. Total length of the extended cavity was about 25 mm. Coarse wavelength tuning from 1130 nm to 1175 nm could be achieved by rotating the modified mirror mount horizontally to change the incidence angle of the grating. The output power of the ECDL at 1156 nm is shown in Fig. 2(a) with filled red circles as a function of operating current. We note that output power of more than 250 mW can be obtained using this ECDL, which is an advantage in generating more power of SHG. To control the temperature of the GC, a small hole was drilled near the diode, and a ten-kΩ thermistor (Thorlabs, TH10K) was glued inside it. The temperature of the GC was stabilized at 23°C using a 30 × 15 mm2 Peltier thermoelectric cooler (Melcor Corp.). Frequency fine-tuning was achieved using a 6.5 × 6.5 × 10 mm3 stacked piezoelectric transducer (PZT) (Thorlabs, AE0505D08F), which is installed at the horizontal adjustment screw of the modified mirror mount. Two AR-coated cylindrical lenses were used to make the output beam near-Gaussian. The first one is a plano-convex cylindrical lens with a focal length of 40 mm (Thorlabs, LJ1402L1-C), and the second one is a plano-concave cylindrical lens with a focal length of −6.4 mm (Thorlabs, LK1087L1-C). The insertion loss was only 4% with this lens set. When the laser beam was coupled into a single-mode fiber (SMF) for 1064-nm wavelength, the coupling efficiency was 65%, which indicates the laser beam profile was successfully converted into near-Gaussian by the cylindrical lens set. An optical isolator was used to prevent the optical feedback from the fiber coupling setup.

To test the mode-hop-free tuning capability, the fiber-coupled ECDL output was sent to a fiber-Mach-Zehnder interferometer (FMZI), which has a path imbalance of 5 m [16

]. While the laser frequency was scanned using the PZT, the interferometer signal was measured using an InGaAs photodiode (PD) with the bandwidth of 50 MHz. This result is shown in Fig. 2(b), in which a clean interference signal extending over 53 fringes can be seen without a frequency jump. As one fringe-spacing corresponds to 40-MHz frequency scan [16

], the mode-hop-free tuning capability was estimated to be more than 2.1 GHz. Although this data was obtained with the output power of about 100 mW, this mode-hop-free tuning capability of more than 2 GHz was maintained at higher output power of over 200 mW. When the optical frequency is doubled by second harmonic generation, the mode-hop-free tuning range is expected to be more than 4.2 GHz at 578 nm.

Next, we measured the frequency noise power spectral density (PSD) of the free-running 1156-nm ECDL, which is shown in Fig. 3

]. While the interference signal of the temperature-stabilized FMZI remained at the quadrature point of the interferometer, the power spectrum of the PD signal after the FMZI was measured with a fast Fourier transform spectrum analyzer. This was converted into the frequency noise PSD using the discrimination slope of 114 MHz/V at the quadrature point. As can be seen in Fig. 3, the frequency noise of the free-running 1156-nm ECDL laser approximately shows a 1/f frequency noise behavior below 40 kHz except for the power-line (60 Hz) harmonics and some broad peaks (around 1 kHz, for example) attributed to the mechanical resonances or the acoustically-coupled noises. The root mean square (rms) linewidth [17

] was estimated to be about 180 kHz by integrating the frequency noise spectrum from 100 kHz down to 1 Hz.

As this laser system will be used as a clock laser for the ytterbium lattice clock, a wide-band frequency modulation is required for the linewidth reduction, thus, an n-channel junction-field effect transistor (JFET) was used for the fast current modulation of the LD by bypassing a small part of the LD current as a function of the applied voltage to the JFET gate. To estimate the modulation bandwidth of this current control by the JFET, the interference signal of the FMZI [16

] was adopted with an acousto-optic modulator (AOM) (150 MHz) inserted in one arm of the FMZI. The interference signal was demodulated by a frequency mixer using the 150-MHz signal, which drove the AOM, and was low-pass-filtered to obtain the error signal for the feedback. When the frequency of the ECDL was stabilized, the noise spectrum at the mixer input was measured by a −10 dB directional coupler and an RF spectrum analyzer, which is shown in Fig. 4

Fig. 4 The noise spectrum at the mixer input, when the frequency of the ECDL was stabilized by a heterodyne FMZI method. The servo bump can be seen at 1.8 MHz, which represents the minimum value of the modulation bandwidth of the JFET current control.

. As can be seen, there was a servo bump at 1.8 MHz, which represents the feedback control bandwidth. The modulation bandwidth of the JFET current control can be estimated to be more than 1.8 MHz from this result.

3. Yellow Light Generation by SHG

The 578-nm yellow light was generated by SHG of the high power 1156-nm ECDL. A WG-PPLN (HC Photonics) was used as a nonlinear medium, which is 0.5-mm thick and 20-mm long. The experimental setup for the SHG is included in Fig. 1. The output of the 1156-nm laser beam was split into two parts controlling the optical power ratio using a half-wave plate (HWP) and a polarizing beam splitter (PBS). One part was coupled to an SMF for the 1156-nm laser output, and the other was focused into a WG-PPNL. The coupling efficiency into the WG-PPLN was measured to be 36%. A HWP was inserted before the WG-PPLN to rotate the polarization direction for the maximum SHG efficiency. A collimation lens was used after the WG-PPLN, and the remaining 1156-nm light was filtered out using a wide-band hot mirror (Thorlabs, FM01), so that only 578-nm output could be separated. As the efficiency of the SHG is sensitive to phase-matching condition, the SHG output power was measured varying the temperature of the oven, which contains the WG-PPLN crystal. The phase-matching curve is shown in Fig. 5

Fig. 5 Output power of SHG by WG-PPLN as a function of the temperature

. From this result, the phase-matching temperature was determined to be 56.2°C and the SHG output power can be maintained within 99% of the maximum power in the course of one day, if the temperature is controlled within 0.1°C, which is not a difficult experimental condition.

Next, the output power of the SHG as a function of the 1156-nm input power was investigated, which is shown in Fig. 6

Fig. 6 Output power of SHG (filled red circles) and the conversion efficiency (open blue circles) as a function of the input power of 1156-nm laser.

. The x-axis in Fig. 6 is the 1156-nm laser power, which was coupled into the WG-PPLN. The filled red circles in Fig. 6 are the SHG output power and can be fitted by a quadratic function, of which coefficient is shown in Fig. 6. This quadratic dependence indicates that the pump depletion was not severe to this power level. We could obtain the maximum 578-nm output power of 10.5 mW, when the 1156-nm laser power before the WG-PPLN was 98 mW (coupled power of 35.3 mW). Although SHG output power of more than 40 mW is expected with the full 1156-nm power of 250 mW, we did not increase the IR power to be more than 100 mW with concern about the optical damage of the WG-PPLN, which is well known in case of proton-exchanged WG-PPLN [8

] is used in future, which has a higher damage threshold, we expect that the full output power of 1156-nm ECDL can be utilized producing much more output power at 578 nm. The SHG conversion efficiency was also calculated, and it is also shown in Fig. 6 with open blue circles. The maximum conversion efficiency was 29.6% with the IR power of 35.3 mW coupled into the WG-PPLN.

As the frequency of the 578-nm laser output will be stabilized to a super-cavity for the linewidth reduction [20

], the short- and long-term frequency variations are of interest. Thus, we measured the short-term linewidth of the free-running frequency-doubled 1156-nm ECDL by using the heterodyne beat spectrum between the SHG of the 1156-nm ECDL and a narrow 578-nm radiation obtained by the SFG of a 1319-nm Nd:YAG laser and a 1030-nm Yb-doped fiber laser, which is shown in Fig. 7

. The full width at half maximum of the beat spectrum was 400 kHz, when the resolution bandwidth of the RF spectrum analyzer was 100 kHz and the sweep time was 2.44 ms. The frequency of the SFG radiation was stabilized to a super-cavity. As the linewidth of this SFG radiation was estimated to be about 80 Hz, the width of the heterodyne beat spectrum (400 kHz) is attributed to the linewidth of the SHG of 1156-nm ECDL. Accordingly, the linewidth of the free-running 1156-nm ECDL can be estimated to be 200 kHz. This result reasonably agrees with that from the frequency noise spectrum in section 2. From these results, the JFET current modulation bandwidth of 1.8 MHz, which was measured in section 2, is expected to be sufficient for the linewidth narrowing using an ultra-stable super-cavity.

Finally, the frequency drift of the 578-nm output was measured using a high resolution wavelength meter (HighFinesse, WS-U-30). In Fig. 8

, the frequency of the SHG of the free-running 1156-nm ECDL was measured for more than 20 hours. As can be seen in the figure, the frequency drift was within 650 MHz in a day. The diurnal variation of the room temperature was typically about 3°C. The frequency oscillation with a period of about 1 hour is attributed to the room-temperature control cycle time. Using the frequency data measured by the high resolution wavelength meter, the relative frequency instability of the free-running frequency-doubled 1156-nm ECDL in terms of Allan deviation [21

with filled red circles. The frequency instability was 5.6 × 10−10 at 1 s of averaging time and reached 1 × 10−7 at 10000 s. The wavelength stability limit is also shown in Fig. 9 with open blue circles, which is well below the measured frequency instability of the SHG of the free-running 1156-nm ECDL. This limit was obtained by measuring the frequency of an acetylene-stabilized laser, which has the frequency stability (Allan deviation) of 1.1×10−12τ−1/2, where τ is the averaging times in seconds, with the same type of wavelength meter in IR range.

4. Conclusion

Yellow light at 578 nm was generated using the SHG of a high power 1156-nm ECDL with maximum power of more than 250 mW. A WG-PPLN was used as a nonlinear medium, and the output power at 578 nm was more than 10 mW, which is the highest output power at this wavelength using SHG of a solid state laser source without an enhancement ring cavity, to our knowledge. This is expected to be an excellent alternative to the system using SFG with the advantage of compactness, simplicity, and cost-effectiveness. The free-running ECDL had the short-term line width of about 200 kHz. The frequency instability was 5.6 × 10−10 at 1 s of averaging time and reached 1 × 10−7 at 10000 s. The frequency drift was within 650 MHz in a day. The JFET current modulation bandwidth was measured to be more than 1.8 MHz, which is expected to be sufficient for the linewidth narrowing using an ultra-stable super-cavity. This system is planned to be used as a clock laser of the ytterbium optical clock.

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